1. Field of the Invention
The present invention relates to a method of leading in APC control for a multibeam optical write method which makes an optical write on an image carrier using a plurality of light beams, and an image forming apparatus using the same.
2. Related Background Art
In general, an image forming apparatus such as a printer, copying machine, facsimile apparatus, or the like makes an optical write on a photosensitive drum using a laser chip that integrates a single semiconductor laser and a single photodiode. More specifically, the semiconductor laser is driven in accordance with an image signal, and a laser beam modulated in correspondence with the image signal is linearly scanned on the photosensitive drum via a polygonal mirror, thereby making an optical write on the photosensitive drum. The photodiode detects some light components of the laser beam emitted by the semiconductor laser, and the detection signal is fed back to control the light amount of the semiconductor laser to a predetermined amount. This control will be referred to as APC control hereinafter.
In such an image forming apparatus, in order to turn on the OFF semiconductor laser and to start its APC control (to lead in the APC control) to print, a drive current is supplied to the semiconductor laser to set it continuously ON while the polygonal mirror is rotating. The light reflected by the polygonal mirror is detected by a sensor, and when a synchronization signal is detected from the output signal of the sensor, the APC control is led in synchronously with the synchronization signal, thus starting the APC control.
In the conventional technique, when an optical write is done in units of lines using a single semiconductor laser, the APC control can be easily led in, as described above. However, when optical writes are simultaneously done on a photosensitive drum by a plurality of laser beams using a plurality of semiconductor lasers, since only one photodiode is equipped in correspondence with the plurality of semiconductor lasers, it is difficult to attain the APC control of the individual semiconductor lasers by simultaneously turning on these semiconductor lasers. For this reason, it is hard to directly use the conventional APC control lead-in method for a single semiconductor laser in a multibeam write apparatus using a plurality of semiconductor lasers, and a new APC control lead-in method for a multibeam optical write is demanded.